You have a very decent PSU. Keep in mind, that if the PSU pulls 1000w from the wall this means your rig is pulling arround (1000 * 94% efficiency = ) 940w. Your PSU should be able to deliver 1200w output, so arround 1275w from the wall. I think you are good to go from that perspective. I guess your PSU is faulty or the connections to the GPUs are bad and the voltage drop there is to big. Do you have any PCIe 8 Pin splitters on it? Maybe their Ohm resistance is too big.